The combustion of air/fuel mixtures in internal combustion engines, such as those found in automobiles, produces an exhaust gas stream comprised of various gaseous components. Some of these components, such as hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NO.sub.x), may be termed noxious components. Those skilled in the art will appreciate that oxides of nitrogen refers to both NO and NO.sub.2. In recent years, environmental concerns have led to increasingly stricter regulations concerning the maximum allowed emissions of these particular components.
Attempts to eliminate or control these noxious components have most recently involved the use of three-way catalysts (TWC) which typically oxidize HC and CO, while simultaneously reducing NO.sub.x. Such a three-way catalytic converter is located in an exhaust line downstream from the engine combustion chamber. With the use of three-way catalysts, the exhaust gas stream typically passes through a combination of precious metals coated onto stabilized alumina and/or rare earth supporting phases carried on a monolithic ceramic or metallic cellular core.
However, optimizing both the oxidation of HC and CO and the simultaneous reduction of NO.sub.x requires close control of the air/fuel ratio entering the internal combustion engine. Optimum reduction of all three components occurs when the air/fuel ratio is close to stoichiometric, i.e., approximately 14.5-14.6 kilograms of air to 1 kilogram of gasoline.
In order to achieve and maintain the desired air/fuel ratio, exhaust gas oxygen (EGO) sensors and closed loop control circuits have been used in conjunction with three-way catalysts. Such emission control systems generally measure the oxygen concentration of the exhaust gas and adjust the relative amounts of air and fuel supplied to the engine in response thereto. EGO sensors provide a feedback assessment whenever the air/fuel ratio is other than a predetermined level, i.e., stoichiometric levels, to indicate whether the mixture is richer or leaner than the stoichiometric level. The stoichiometric level is known as the closed loop control point, and within narrow limits, may be selected as desired.
While the use of three-way catalysis has been found to be useful, its accuracy and effectiveness is impaired with the use of various alternative automotive fuels which results in a significant lean shift of the closed loop control point. It is believed that the low molecular weight of hydrogen allows it to preferentially diffuse through the protective spinal sheath surrounding the electrode of the typical EGO sensor. As a result, the relative hydrogen concentration at the electrode surface is higher than the relative hydrogen concentration at the outer surface of the sheath. This higher relative concentration causes the EGO sensor to detect a richer gas mixture than is present. As a result, the EGO sensor experiences a "shift." Alternative fuels are generally designated as those other than traditional gasoline and diesel fuels. Illustrative examples are natural gas, methanol, methane, propane (LPG), ethanol and combinations of these fuels.
In general, lean shifts result in inaccurate control of the air/fuel mixture entering the engine. That is, because the EGO sensor believes the air/fuel mixture to be richer in fuel than it is actually is, the closed loop control system directs a reduction in the amount of fuel entering the engine. The air/fuel mixture entering the engine thus becomes leaner than what is actually called for, resulting in a lean shift.
U.S. Pat. No. 5,433,071 to Willey et al. and assigned to Ford Motor Company, incorporated for reference herein, discloses An Apparatus And Method For Controlling Noxious Components Using A Conditioning Catalyst For Removing Hydrogen. Under the '071 patent, a conditioning catalyst removes hydrogen from the exhaust stream and an EGO sensor, in communication with the conditioned exhaust gas, generates a signal in response to the sensed oxygen concentration of the conditioned exhaust gas. The air/fuel mixture entering the engine is adjusted by a closed loop control means in response to the signal generated by the EGO sensor. The removal of hydrogen from the exhaust gas stream eliminates lean shifts and allows the oxygen sensor and closed loop control means to accurately control the air/fuel ratio.
One of the key benefits of this conditioning catalyst approach is extremely robust emissions durability. The benefits of a conditioned exhaust stream are equally important for gasoline vehicles. However, one significant difference between natural gas as compared to gasoline vehicles is that cold-start hydrocarbon emissions are not a particular concern, primarily, because natural gas consists predominantly of unburned fuel (methane) which is excluded from the hydrocarbon standard. Moreover, since natural gas is a gaseous fuel, it has none of the cylinder-wall wetting problems attributed to cold-start emissions with liquid fuels such as gasoline. Given the durability benefits of the conditioning catalyst in natural gas fuels, there is a need for achieving such benefits with the use of gasoline vehicles.
There is thus a need for a catalytic assembly which will achieve very low emissions of noxious components like NO.sub.x, HC and CO with a liquid fuel such as gasoline, while exhibiting long term durability and little or no loss in efficiency.
In addition, further research has indicated that a small calibrated reduction of oxygen occurs in addition to hydrogen before the main catalyst and is useful to tolerate greater engine combustion variability and optimize air/fuel control. Cylinder-to-cylinder air/fuel variability or engine bank-to-bank air/fuel variability beyond an acceptable amount manifests itself in a lean shift in the bulk exhaust gas composition such that desired catalyst efficiencies cannot be realized. This occurs whether a particular cylinder is either rich or lean. As provided in the '071 patent, a low-volume catalyst placed before the controlling HEGO sensor removes some of the oxygen and other reactive gases which effects partial gas equilibration and reduces the emission impact of cylinder-to-cylinder or engine combustion variability. There is still a further need to eliminate lean shifts in the bulk exhaust gas composition so that desired catalyst efficiencies can be realized.
In addition, there is also a further need to reduce the reactive gas load on the catalyst system for a particular component of the catalyst system.